The copending U.S. patent application of D. M. Klein et al, Ser. No. 357,848, now U.S. Pat. No. 4,425,719, filed Mar. 15, 1982, which has a common assignee with this application, discloses web drying apparatus comprising air bar assemblies of unprecedented compactness. The present invention is concerned with an improvement which is particularly useful in web dryers comprising such air bar assemblies.
An air bar assembly made in accordance with that patent application comprises an elongated header which extends lengthwise of a stretch of web to be dried and which has transversely extending air bars at regular intervals along its length, each communicated with the interior of the header through a port midway between its ends. Each such air bar assembly has its own blower, located at a rear end of the header, by which pressure air is forced into the header for emission from the air bars along it. There is one such assembly below each stretch of web, and another one above the stretch. From the air bar assembly below the web stretch air issues upwardly against the stretch to levitate it, while the stretch is steadied and guided by air issuing downwardly against it from the air bar assembly above it. The air streams thus directed against the web effect rapid drying of it.
An air bar assembly of said copending application comprises pressure air distributing means for ensuring uniform emission of air from all air bars along the length of the header, notwithstanding that some of the air bars are mounted on the rear portion of the header, directly opposite the blower, whereas others are at the front end of the header and thus at some distance from the blower. Reference can be made to that application for all details of this pressure air distributing means.
For optimum compactness and versatility, air bar assemblies of the copending application are preferably incorporated in a dryer that accommodates two or more horizontal stretches of web, disposed one above the other. The respective stretches may be those of different webs, or one web may be guided along a sinuous path to make repeated passes through the dryer enclosure in which the air bar assemblies are housed. In either case, vertical height of the dryer is minimized by arranging the two air bar assemblies that are between web stretches with their headers in front-to-rear relationship to one another so that their blowers are lengthwise remote from one another. With the air bar assemblies so disposed, a dryer enclosure that compactly houses them has relatively little space in its interior to accommodate fuel burners by which air is heated and ducting by which the heated air is conducted to the several blowers. Furthermore, if there is a burner for every air bar assembly, to provide for individual control of the temperature in each header, the rear wall of the dryer enclosure, on which the burners and their controls are mounted, becomes extremely cluttered, and there may be a certain amount of difficulty in finding room for all of the burners.
It is apparent that these problems can be overcome by an arrangement wherein one burner serves as a source of heated air for two air bar assemblies, and particularly the two air bar assemblies that are between web stretches, inasmuch as there is a minimum of space between and around those two assemblies. Using one burner for those two air bar assemblies has the obvious further advantage of saving the cost of a fuel burner.
However, sharing the output of one burner between two air bar assemblies gives rise to certain problems. Usually, the air directed against the lower one of a pair of web stretches in a dryer enclosure should be at a higher temperature than the air emitted against the upper stretch, although there can be times when it is desired to maintain the same air temperature at both stretches. This poses the problem of how to have cooler air in the header of the lower air bar assembly for the upper web stretch than in the header of the upper air bar assembly for the lower stretch, when both of those headers are supplied with heated air from the same burner. The problem is complicated by the fact that it must be possible to maintain a desired temperature in each of those two headers, even though the desired temperatures may differ substantially from one another, and even though the temperature to be maintained in each header must be adjustably variable independently of the temperature to be maintained in the other. As brought out in the introductory paragraphs of U.S. Pat. No. 3,151,954, issued to B. C. Ege in 1964, the problem is further complicated by the requirement that changing the temperature in a particular header must not bring about a change in the rate of air flow through the header.